The 85 mm air defense gun M1939 (52-K) (Russian: 85-мм зенитная пушка обр. 1939 г. (52-К)) was an 85-mm Sovietair defense gun, developed under guidance of leading Soviet designers M. N. Loginov and G. D. Dorokhin. This gun was successfully used throughout the German-Soviet War against level bombers and other high- and medium-altitude targets. In emergencies they were utilized as powerful anti-tank weapons. The barrel of the 52-K was the basis for the family of 85-mm Soviet tank guns. Crews of 85-mm AD guns shot down 4,047 Axis aircraft. The mean quantity of 85-mm ammunition required to shoot down one enemy plane was 598 rounds. After the war some 52-Ks were refitted for peaceful purposes as anti-avalanche guns in mountainous terrain.

Virtually every country behind the Iron Curtain received this gun after World War II for their air defense. In the Soviet Union itself, these guns were largely superseded by the 100 and 130 mm guns.

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Adopted in 1939, the 85-mm M1939, like its counterpart the 88-mm gun, was meant for air defense. Like many anti-aircraft (AA) guns of the era it was provided with antitankammunition in the event a tank should appear.

Petrov's bureau developed the D-5 85mm gun. Though much too large for the T-34 turret, it could be mounted in the chassis of the SU-122 self-propelled gun to create a tank destroyer. This was done, and the SU-85 tank destroyer went into production. Meanwhile, efforts were underway to redesign the T-34 turret to make it large enough to accommodate the newly developed gun.

Grabin, who had been working on a second 85mm gun, the ZiS-53, at Joseph Stalin Factory No. 92 in Gorky near Moscow, was reassigned to the Central Artillery Design Bureau (TsAKB) in Moscow. His project was turned over to 23-year-old A. Savin. A third team led by K. Siderenko was assigned to yet another 85mm gun project, and developed the S-18, to be used in the modified T-34 turret.

The resulting guns were tested at Gorokhoviesky Proving Grounds near Gorky, with Grabin's ZiS-53 winning the competition. However, the new T-34/85's turret had been designed to accommodate the already-available D-5 gun, and did not mate properly with Grabin's gun. The T-34/85 tank went into production making use of the D-5 gun (designated D-5T, for "tank").

Savin was put to work modifying Grabin's gun to fit the new turret and incorporate other improvements. His initial was added to his gun's designation in recognition of his contribution: the ZiS-S-53. From T-34/85 Model 1944 onwards, all T-34/85s used Savin's ZiS-S-53. It went into production in the spring of 1944. Later tests against captured German tanks found that the tank gun developed from the 85-mm AA gun, though an improvement over the 76 mm gun, still lacked the firepower of its German competitors. A new antitank gun was then developed, the 100 mm field gun M1944 (BS-3).